AFTER two years of preparation, planning and construction work, a new government primary school will open in May at Tseung Kwan O in Sai Kung. The school, located at Hau Tak Estate, has 30 classrooms and four special rooms. ''The new school will run whole-day classes when it opens,'' said a spokesman for the department. At present, the area already has a government secondary school. The opening of the primary school will broaden the choice for parents and meet the demand for places. It will also provide a better feeder linkage between the primary and the secondary school operated by the Government, said the spokesman. After the opening of the new school, Kai Tak Government Primary School will cease to operate in the summer of 1994. The vacated premises will be made available for allocation to a sponsoring body for the operation of an aided primary school. ''The provision of primary school places in Wong Tai Sin District will not be affected,'' the spokesman said. In line with other whole-day public sector schools, an enhanced staffing ratio and an improved senior teacher ratio will apply to the new primary school in Tseung Kwan O.